Arachnoid Cyst

            Arachnoid cysts are CSF-containing cysts that are found in the middle fossa, posterior fossa, suprasellar cistern, or near the vertex. They are benign but slowly grow as they accumulate fluid, compressing normal brain structures. Remodeling of the adjacent skull is an important clue for a benign expansile process.

            Arachnoid cysts are smoothly marginated and homogeneous. They are not calcified and do not enhance. The multiplanar capability of MR is particularly helpful in establishing the exact location, and the diagnosis is supported by the cyst fluid being isointense with CSF on all pulse sequences. The cysts may appear higher signal than CSF on intermediate T2-weighted images. The exact reason for this is uncertain, although it may reflect dampening of the CSF pulsations that normally results in signal loss in the ventricles and cisterns. This effect will be less apparent with pulse sequences that incorporate flow compensation techniques. 

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